The invention refers to an actuating device for a double seat valve with cleanable seat according to the preamble of claim 1.
A generic actuating device for a double seat valve with cleanable seat is known from WO 97/22821. This solution is an installation friendly and space saving actuating mechanism, in which the main adjustment mechanism and the individual adjustment mechanisms are combined in a common drive housing and externally adjustable partial stroke limitations for both closure members are provided.
The actuating devices for such double seat valves have to be designed such that they are able to transfer the closure members to an open position against product under pressure present in the drive housing, if applicable, on the one hand, and to bring it, on the other hand, as a counter move to the closed position and to reliably seal it there against the product pressure on their seating surface. If the two closure members are each a seating plate (WO97/22821, FIG. 1), a force generated by the pressure in the associated valve housing acts upon the independently actuated closure member (in the actual case this is, referred to the illustration position, the closure member situated at the bottom) in its opening direction. Thereby, the closing force of the actuating device usually locking by elastic spring force acting upon the seating surface is reduced. In the design of the spring closing forces such influences are either considered or the closure member in question is provided with a so-called pressure compensating piston, which can be dimensioned such that no or no substantial axial forces resulting from the product pressure act upon the closure member (closure member 4 according to FIG. 1 in the WO 97/22821). Above that, such a pressure compensating piston secures the double seat valve also against being pushed open by shortly acting pressure impacts in the associated valve housing, which can constitute a multiple of the nominal product pressure.
If the independently actuated closure member named above is formed as a valve piston, the same measures are taken in order to secure against pushing open and pressure impacts (WO 97/22821, FIG. 6).
The dependently actuated closure member (in the present case this is, relating to the illustration position, the closure member situated at the top), which is designed as seating plate in the double seat valve according to WO 97/22821 in the embodiment according to FIG. 1 as well as according to FIG. 6, is, in the embodiment shown in FIG. 1, pressed onto the associated seating surface with a force arising from the respective pressure situation due to product pressure acting as planned, but also due to pressure impacts occurring unplanned additionally to the locking force, which results from the initial stressing of the spring in the actuating device. In the course of this the product pressure in the associated valve housing weighs on the whole surface in question of the closure member such that the actuating device besides the locking force for securing a sufficient sealing effect in the valve seat also has to provide the additional locking force resulting from product pressure and closure member surface. This is implemented by a corresponding design of the associated actuating piston acted upon by pressure medium. As the operating pressures of the pressure medium to be provided as a rule have a certain standardised magnitude, which cannot simply be enlarged, the adjustment of the actuating device regarding the opening forces to be provided is effected by a corresponding design of the diameter of the actuating piston. A large diameter of the actuating piston and therewith of the actuating device results in turn in higher costs, which, as a rule, increase more than linearly proportional with growing diameter.
In contrast, in the embodiment according to FIG. 6 (WO 97/22821) the additional closing forces resulting from the product pressure are compensated due to the pressure compensating piston arranged at the closure member in question, such that the opening of the associated closure member against the product under pressure present in the associated valve housing is generally eased by the provision of the pressure compensating piston.
The problem described above arises on the one hand during the seat cleansing of the dependently actuated closure member, which in the course of this has to be lifted by a partial stroke against the product under pressure present in an associated valve housing. Therefore, the respective piston of the associated individual adjustment mechanism has to be dimensioned according to the criteria described above. Furthermore, this problem also occurs during each opening stroke of the double seat valve. The opening movement is launched by the actuation of the independently actuated closure member, which is not subject to the problem described above. However, after a partial stroke it comes to rest against the dependently actuated closure member and transfers the latter also into the opened position during its further opening movement. Accordingly the same dimensioning problem arises for the actuating piston of the independently actuated closure member, as the opening movement and accordingly the force variety after abutment of the independently actuated closure member at the dependently actuated one are substantially determined by the pressure situations at the latter.
The contexts described above are a general problem and not only a problem, which occurs in a double seat valve according to WO 97/22821. Also the locking valve described in DE 195 48 860 A1, for example, is subject to the same problem, which is identically designed regarding the closure member configuration and the provided pressure compensation measures as the double seat valve according to WO 97/22821 in the embodiment used there according to FIG. 1. In this known locking valve the easier control and overcoming of the closing forces resulting from the product pressure are not concerned, but the considerations head towards a different direction. It is proposed among other things to increase the contact pressure of the top valve plate in the course of the seat cleansing of the bottom valve plate in that an actuating piston of the individual adjustment mechanism associated with the top closure member is additionally acted upon with pressure in closing direction.
As it can be recently observed that especially globally represented operators of large plants of the food and beverage industry rather provide or favour relatively low pressures of the pressure medium to be provided, the question arises regarding an optimum and with it low-cost design of actuating devices, which shall be operated with these pressures.
The arrangement of a pressure compensating piston at the dependently actuated closure member, as proposed in the embodiment according to FIG. 6 of WO 97/22821, is not in every case the solution of the first choice. With predetermined pressure of the pressure medium, such a construction results in smaller diameters of the actuating pistons and accordingly of the actuating device in comparison to a solution without pressure compensating piston, but pressure compensating pistons comprise sufficiently known disadvantages, especially if they have to extend into the associated valve housing by the distance of the opening stroke due to kinematic reasons. Firstly, such pressure compensating pistons narrow the flow cross section in the valve housing, into which they engage, such that the velocity losses increase substantially, secondly, the pistons having necessarily diameters of large dimensions create sealing problems and hygienic problems in the area of their valve housing passage.